1. Field of the Invention
The present invention relates to an exposure apparatus and an exposure method using that exposure apparatus. Particularly, the present invention relates to an exposure apparatus that allows negative type and positive type exposure using the same mask and that allows multi-focus points and multi-exposure with improved resolution in exposure, and an exposure method using that exposure apparatus.
2. Description of the Background Art
In recent years, semiconductor integrated circuits of large capacity and multiple functions can be obtained economically by combining and forming critical and complex patterns. Advance in the manufacturing technique is continuously sought for reducing the pattern dimension within a semiconductor integrated circuit.
Critical patterns in the manufacturing process of a semiconductor integrated circuit is achieved by photolithography. This photolithography technique requires a resist film to be formed on a semiconductor substrate.
This resist film is made of organic macromolecule that reacts with light, and is formed as a uniform thin film by a spin coating method or the like on a semiconductor substrate that will be subjected to a patterning process. The resist film is subjected to thermal treatment, whereby the organic solvent in the resist film is evaporated, the stress in the resist film removed, and adhesion of the resist film to the semiconductor substrate increased.
Then, the face of the resist film is exposed by light set to a wavelength for the resist film to be photosensitized. Exposure is carried out where an original image (mask) having a predetermined circuit pattern formed is directly adhered to the resist film, or where a projection method is used to form an image of the pattern of the mask onto a semiconductor substrate.
The exposed resist film absorbs light energy to exhibit chemical change. The causes a change in the molecular weight of the resist film. There are two types of the molecular weight change, one in which the macromolecule chain of the resist film composition material is severed to be reduced in molecular weight, and the other in which cross linking reaction occurs between macromolecule chains to be increased in molecular weight.
The former is called a positive type resist having the portion exposed to light removed in the developing process. The latter is called a negative type resist having the portion exposed to light remaining in the developing process.
In current technique, a positive type resist is characterized in that a defect is easily generated but the pattern accuracy is good. The negative type resist is characterized in that a defect is not easily generated but the pattern accuracy is poor. The resists of respective types are used according to their application.
The structure of a conventional exposure apparatus will be described hereinafter with reference to FIG. 17.
Referring to FIG. 17, an exposure apparatus includes a mercury lamp 11 which is the light source, a reflecting mirror 12, a focused lens 20, a fly eye lens 13, a diaphragm 14, focused lens 16a, 16b, 16c, a diaphragm 15, a reflecting mirror 17, a photomask 18, projection lens 19a, 19b, and a diaphragm 25.
The light 11a emitted from mercury lamp 11 has only the g-line (436 nm), for example, reflected by reflecting mirror 12 to result in a unit wave length. Light 11a enters each fly eye forming lens 13a of fly eye lens 13 to pass through diaphragm 14.
In FIG. 17, light 11b indicates a light path generated by one fly eye forming lens 13a, and light 11c indicates a light path generated by fly eye lens 13. Light 11a passes focused lens 16a, diaphragm 15, and focused lens 16b to reflecting mirror 17, whereby only light 11a of a unit wave length of g-line (436 nm) is reflected.
Then, light 11a passes focused lens 16c to be directed in uniformly to the surface of photomask 18 having a predetermined pattern formed. Light 11a is reduced to a predetermined magnification by projection lens 19a and 19b to expose a resist film 21a on a semiconductor wafer 21.
In the case of a positive type resist film, the portion exposed to light will be removed during the developing process. In the drawing, light 11d indicates diffraction light of zero degree, and light 11e and light 11f indicate diffraction light of .+-. one degree.
A trend for a more critical exposure dimension is recently noticeable. The need arises for further improving the pattern resolution in a semiconductor device. A phase shift method is known for this purpose in which a phase difference is established for light passing a photomask to improve the light intensity profile. Such a phase shift method will be described hereinafter with reference to FIG. 18A-18D and FIGS. 19A-19D.
Referring to FIG. 18A, a conventional photomask 18 for forming a line-and-space pattern includes a light shielding portion 18d formed using light shielding material such as chrominium, molybdenum, or silicide, and a light transmitting portion 18c on a transparent mask substrate 18a such as of quartz substrate. A repeated pattern of a line-and-space is formed to serve as a mask pattern 18 for exposure.
The intensity distribution of light (A1) right after passing resist pattern 18 can be appreciated from FIG. 18B where light is 0 at light shielding portion 18d, and light completely passes through light transmitting portion 18c.
Taking one light transmitting portion 18c as an example, the light (A2) provided to the resist film has an amplitude with a hill-like maximum at both trailing edges thereof due to diffraction as shown in FIG. 18C. The light passing through an adjacent light transmitting portion 18c is shown by the chain dotted line A2'.
When light from each light transmitting portion 18c is gathered, the light intensity distribution will lack sharpness as shown by the solid line A3 in FIG. 18D. This will cause an unsharp image due to diffraction, so that a sharp exposure can not be accomplished.
This problem can be solved by providing a phase shifter 23 on every other light transmitting portion 18c of the repetitive pattern as shown in FIG. 19A. Unsharpness due to diffraction is canceled by inversion of the phase, whereby a sharp image is transferred to improve the resolution.
When phase shifter 23 provided on one light transmitting portion 18c is such that applies a phase difference of 180.degree., for example, to light, the light passing through phase shifter 23 is inverted as shown in B1 in FIG. 19B. The light passing through light transmitting portion 18c is not inverted since it does not go through phase shifter 23.
The light applied to the resist film will be inverted to cancel each other at the edge of the light intensity distribution thereof at a position indicated by B2 in FIG. 19C. Therefore, the light distribution applied to the resist film will exhibit an ideal shape as shown by B3 in FIG. 19D.
This method of providing a phase shifter 23 at every other line-and-space is called a Robinson type phase shifter method.
Another conventional example using a phase shifter method will be described hereinafter.
An auxiliary shifter type phase shifter method will be described with reference to FIGS. 20A-20E.
Referring to FIG. 20A, a photomask is providing including a light transmitting portion 18c and light transmitting portions 18e and 18e of a small opening width at both sides of light transmitting portion 18c on a quartz mask substrate 18a. A phase shifter 23 applying a phase difference of 180.degree. is provided at light transmitting portions 18e, 18e.
When light is directed to photomask 18 of the above-described structure, the light passing through light transmitting portion 18c has an amplitude of a mountain shape with extending trailing edges as shown in FIG. 20B. The light passing through phase shifter 23 shows an amplitude of two small mountains on the resist film as shown in FIG. 20C.
The light intensity on the resist film results in the overlap of the 2 light amplitudes as shown in FIG. 20D. The light distribution shows a sharp mountain configuration with the extension of the edges suppressed, so that unsharpness due to diffraction is canceled. It is therefore possible to form a resist film 21a having a sharp image on a semiconductor wafer 21 as shown in FIG. 20E.
A self-alignment type phase shifter method will be described hereinafter with reference to FIG. 21A-21E.
Referring to FIG. 21A, a light transmitting portion 18c and a light shielding portion 18d are formed on a quartz mask substrate 18a. A phase shifter 23 applying a phase difference of 180.degree. to the transmitting light is provided at the boundary of light transmitting portion 18c and light shielding portion 18d.
When light is directed to photomask 18 of the above-described structure, the light passing through light transmitting portion 18c shows an amplitude of a mountain with extending edges on a resist film as shown in FIG. 21B.
The light passed through phase shifter 23 shows an amplitude of two small mountains with a gentle slope at the trailing edges on a resist film as shown in FIG. 21C.
Therefore, the light intensity on a resist film results in the overlap of the above-described two light amplitudes. The light intensity distribution shows a sharp mountain with the extension of the trailing edges suppressed.
Thus, a resist film 21a having a sharp image can be formed on a semiconductor wafer 21, as shown in FIG. 21E.
A halftone type phase shift method will be described hereinafter with reference to FIGS. 22A-22E.
Referring to FIG. 22A, a light transmitting portion 18c and a light shielding portion 18d are formed on a quartz mask substrate 18a. Light shielding portion 18d is formed of a material that transmits approximately 10% of the light. A phase shifter 23 providing a phase difference of 180.degree. with respect to the transmitting light is provided on the upper face of light shielding portion 18d.
When light is directed to a photomask 18 of the above-described structure, light passing through light transmitting portion 18c shows an amplitude of a mountain configuration with extending edges on a resist film, as shown in FIG. 22B.
Approximately 10% of light going through light shielding portion 10d results in an amplitude of light having the phase inverted 180` on a resist film, as shown in FIG. 22C.
The light intensity on the resist film results in the overlap of the above-described two light amplitudes, as shown in FIG. 22D. It can be appreciated that light intensity is suppressed in the trailing edge of the mountain although approximately 10% of light intensity is seen at both edges.
It is therefore possible to form a resist film 21a having a sharp image on a semiconductor substrate 21 as shown in FIG. 22E.
Although the resolution is equal, the halftone type phase shift method is advantageous over the auxiliary shifter type and self-alignment type phase shifter method in that a complex phase shifter 23 does not have to be formed. A resist film can easily be formed just by providing a phase shifter 23 on a light shielding portion 18d transmitting approximately 10% of light.
A phase shift method of a shifter light shielding type I will be described hereinafter with reference to FIGS. 23A-23E.
Referring to FIG. 23A, a phase shifter 23 providing a phase difference of 180.degree. to the transmitting light is provided at a predetermined interval on a quartz mask substrate 18a.
When light is directed to photomask 18 of the above-described structure, the light passing through the light transmitting portion 18c of quartz mask substrate 18a shows an amplitude reduced at portions corresponding to phase shifter 23 on the resist film, as shown in FIG. 23B.
The light passing through phase shifter 23 shows an amplitude having the phase inverted 180.degree. on the resist film, as shown in FIG. 23C.
The light intensity on the resist film results in the overlap of the above-described two light amplitudes, where light intensity of substantially 0 is seen at the portions corresponding to each phase shifter 23, as shown in FIG. 23D.
Therefore, it is possible to form a resist film 21a having a configuration corresponding to phase shifter 23 on a semiconductor substrate 21, as shown in FIG. 23E.
A phase shift method of a shifter shielding light type II will be described hereinafter with reference to FIGS. 24A-24E.
Referring to FIG. 24A, a phase shifter 23 of a predetermined width and providing a phase difference of 180.degree. with respect to the transmitting light is provided on a quartz mask substrate 18a.
When light is directed to photomask 18 of the above-described structure, light passing through light transmitting portion 18c on quartz mask substrate 18a will exhibit an amplitude as shown in FIG. 24B on a resist film. The light passing through phase shifter 23 exhibits an amplitude having the phase inverted 180.degree. on the resist film, as shown in FIG. 24C.
The light intensity on the resist film is the overlap of the above-described 2 light amplitudes, as shown in FIG. 24D. The light intensity becomes substantially 0 at the portions corresponding to the opposite end portions of phase shifter 23.
Thus, it is possible to form a resist film 21a only at the areas corresponding to the two end portions of phase shifter 23, as shown in FIG. 24E.
A multi-step type phase shift method will be described hereinafter with reference to FIGS. 25A-25D.
Referring to FIG. 25A, a phase shifter 23b of a predetermined width providing a phase difference of 180.degree. with respect to the transmitting light, and a phase shifter 23a providing a phase difference of 90.degree. with respect to the transmitting light at one side of phase shifter 23b are provided on a quartz mask substrate 18a. There are transmitting portions 18c at both sides of phase shifters 23b and 23a.
The phase of light going through photomask 18 of the above-described configuration reaching the resist film is shown in FIG. 25B. The phase of light passing through phase shifter 23b, phase shifter 23a, and light transmitting portion 18c are indicated by (i), (ii), and (iii), respectively.
It can be appreciated from FIG. 25C showing the light intensity on the resist film that there is a portion that is not exposed at the interface of phase shifter 23b and light transmitting portion 18c caused by the light intensity reduced to substantially zero.
In contrast, the portion including phase shifter 23b, phase shifter 23a and light transmitting portion 18c will be exposed because the light intensity is never reduced to 0 on account of phase shifter 23a.
Thus, a resist film 21a as shown in FIG. 25D is formed in using photomask 18 of the above-described structure.
The problems of the above conventional methods are set forth in the following.
When a pattern identical to that of the photomask is to be transferred to a resist film, a positive type resist film is used. When the pattern of the photomask is inverted to be transferred to a resist film, a negative type resist film is used.
A positive type resist film is formed of a novolac resin type resist, and a negative type resist film is formed of a gum type resist. There is much constraint in usage thereof due to difference in their handling manners, effective developers, and developing methods.
When a projection type photomask is used as in the above-described conventional art, the exposure was adversely affected by defects found at the top and bottom surfaces of the photomask. The defect in the photomask had to be located and corrected which took a great period of time.
When a phase shift mask is used, it is difficult to find a defect since the phase shifter is formed of a transparent material. Furthermore, the manufacturing process of a phase shifter is a difficult one. When the pattern is irregular, the manufacturing process of the phase shifter would become very complicated.
Furthermore, there was a problem that a resist film could not be etched in a desired configuration when a stepped portion is generated on the semiconductor substrate on account of the focusing position differing within the same semiconductor substrate.
Exposing a resist film 51 on a semiconductor substrate 50 having a stepped portion will be described with reference to FIGS. 26A-26C.
FIG. 26A shows the case where the focusing position is set based on the resist film on a higher face portion 50a of semiconductor substrate 50 as the reference.
A hole 52 of a predetermined configuration can be formed in a resist film 51 on higher face portion 50a. In contrast, a hole 53 of imperfect configuration is formed in resist film 51 on the lower face portion 50b because that region is out of focus.
FIG. 26B shows the case where the focusing position is set to resist film 51 on lower face portion 50b of semiconductor substrate 50.
In this case, hole 53 of a predetermined configuration can be formed in resist film 51 on region 50b. However, hole 52 can not be formed with a desired configuration in resist film 51 on higher face portion 50a because the area does not have the apparatus depth of focus.
FIG. 26C shows the case where the focusing position is set to an area intermediate the portions of 50a and 50b in semiconductor substrate 50. In this case, holes 52 and 53 are formed imperfect in configuration because neither are in focus.
There is also a problem that holes of desired configuration can not be obtained informing holes of different depth in resist film 51 on the same semiconductor substrate. This is due to the fact that the exposure (amount of light) for forming respective holes differs.
This problem will be described in detail with reference to FIGS. 27A-27C.
FIG. 27A shows the case where the exposure is set based on a hole 52 smaller in depth. In this case, hole 52 will be formed in a predetermined configuration, but hole 53 will be imperfect in shape because the exposure is insufficient.
FIG. 27B shows the case where the exposure is set based on the deeper hole 53. In this case, hole 53 will be formed in a predetermined configuration, but hole 52 will be imperfect in configuration due to excessive exposure.
FIG. 27C shows the case where the exposure is set based on an intermediate depth of holes 52 and 53. In this case, holes 52 and 53 will both be imperfect in configuration.